The changed frame conditions on the electricity market where leading to employ hydropower plants more and more often to compensate fluctuations in the electricity grid. The resulting operation in part load makes it necessary to know critical operating points, to be able to avoid them. A well known example for such a critical operating point is the resonance rotational speed, here the machine is subjected to high loads, wich can lead to its destruction. The aim of this work was to determine the effect of surrounding water on the natural frequencies of a Francis-runner by using the finite element method (FEM). For this purpose special acoustic elements where used, which made it possible to simulate the propagation of pressure waves in the fluid. Also the general influence of the water, better known as added mass effect, is shown. Furthermore the influence of the water volume size is investigated as well. Another attention was directed on modeling the water volume. Overall three various models where used, whose results where compared among each other. At last the results of one of these models is validated with measurment data. This data originates from a strain measurement wich was performed in various operation points. In these operation points several phenomens occur, wich are able to excite vibrations in the runner.